Colloidal solutions are between a true solution and a suspension. They are a dispersion of particles in a liquid medium where the size of the particles (or “colloids”) is between about 1 and about 1000 nanometers. They include, but are not limited to, sols, and aqua-sols or hydrosols of metal oxides (or precursors thereof). Sols, in particular, are colloidal solutions containing particles between about 1 and about 100 nanometers in size.
Often when a colloidal solution is prepared, it is present in a relatively dilute concentration for ease in handling. Subsequent processing techniques to which the colloidal solution may eventually be subjected, however, may necessitate that the colloidal solution be concentrated.
Current methods of concentrating colloidal solutions are by sedimentation, filtration, centrifugation, and various evaporation method including batch evaporation or by continuous evaporation using parallel plate evaporators, extruders, wiped film evaporators, thin film evaporators, rotoevaporators, rising film evaporators and falling film evaporators. A problem with these known evaporators is that they are particularly difficult to use with materials that show large increases in viscosity with small changes in concentration, as is typical of colloidal solutions.
Another problem with the known evaporators is that the colloidal solution may be over-evaporated. Over-evaporation may occur for more than one reason. For instance, one reason is that as the colloidal solution is being concentrated, it is often in contact with a heated or hot surface in the evaporation zone. This may result in over-evaporation. When something is “overevaporated,” it is more evaporated than desired. Over-evaporation may also be the result of the known evaporators not having sufficient control to precisely evaporate a specific amount of material. A consequence of over-evaporation is that it may result in some of the colloidal solution being degraded (decomposed, e.g. burned). The degraded remains of the colloidal solutions may foul the evaporation equipment and the concentrated colloidal solution. Concentrated colloidal solutions substantially lacking such contaminates are necessary for certain ultimate uses of the materials. Over-evaporation may also result in premature gellation of the colloidal solution.
Another problem with known evaporators is under-evaporation of colloidal solutions. When something is “underevaporated,” it is less evaporated than desired. A negative result of under-evaporation is, for example, that the resultant article formed from the under-evaporated colloidal solution may not be able to hold its desired shape.
Current processes generally use evaporator temperature to control the evaporation to reach the desired level of evaporation of the colloidal solution. For example, evaporation may be done by rotoevaporation, which involves evaporating liquid from a heated, rotating vessel into a cooled receiving flask. The problem with using temperature to control the amount of evaporation is that the system is slow to respond to a change in temperature.